The most common optical waveguide is the fiber with a round-shaped core supported by a round-shaped cladding. The next common optical waveguide is a planarized waveguide in which light-guiding channels are fabricated near the surface of an usually right-angled optical substrate. Guided light resides mostly inside the core. The sectional dimension of the fiber core is typically less than ten microns (0.01 mm) in single-mode fibers, and usually less than 200 microns even in the multimode fibers. Accordingly, connecting and coupling (mixing) of light between two or more fibers present enormous technical challenge. As a result, the prices for connectors and couplers for optical fibers are quite expensive, especially when compared to the counterpart components for microwave cables. Since the connectors and couplers are among the most frequently used components in the optical fiber communication, the high component price has impeded the expansion of the optical fiber communication into the broader applications, such as picture-phone, computer networking, and cable television.
Planarized waveguides are patterned on the flat top surface of a bulk optical substrate using microlithography. Accordingly, many of the useful optical functions, such as core tapering and light coupling, may be realized on planarized waveguides.
On the other hand, the fiber is drawn from molten glass or plastic materials into a hair size and shape at a high speed. Accordingly, it is not easy to incorporate tapering and light coupling functions into the optical fiber body. To make a fiber coupler, it is necessary to perform an additional fabrication with strands of fiber. The first multimode fiber star coupler or light mixer available in early 1970's comprises one linear array of fibers butted against the one side of a narrow and long rectangular cavity, and the other linear array butted against the opposite side of the cavity. Light from any of the fibers spreads inside the cavity while propagating the long length, and uniformly illuminates the array of the fibers on the opposite side. Advantage of this coupler is the simple construction and wavelength independence of the coupling ratio. The drawback of such a fiber star coupler is that the core occupies only a fraction of the sectional area due to its roundness and the existence of the cladding around each core. Accordingly the excess loss, due to the so-called packing density factor, is inherently high even with multimode fibers, and cannot be used at all for the single-mode fibers due to the extremely small core-to-cladding area ratio, which is about one to a hundred.
An alternative approach, so-called biconical fused coupler, was used from early 1970's for multimode fibers, in which fibers are twisted, thermally softened, and pulled very slowly, until the light guided in the core leaks out of the core into the cladding due to the size decrease in the pulled-and-fused area. Once the light resides in the cladding area it is freely spread into the claddings of other neighboring fibers, that have been fused together in the pulling process. The cladding modes return to the cores as the fiber sizes increase in the second half of the fused section. This process relies on the taper being gradual and smooth, ensuring an adiabatic mode transformation between the core modes and cladding modes. As the single-mode fiber began its dominance over the multimode fiber in the interferometric fiber sensor area starting about 1975, the requirement for single-mode coupler emerged. However, the processing technology of the biconical fused coupler mentioned above was not good enough to produce single-mode fiber coupler in that period. The first published single-mode star coupler was made by Sang K. Sheem, one of the inventors of the present invention disclosure, by twist-and-etch technique, as disclosed in U.S. Pat. No. 4,264,126. For a few years in the late 1970's, this coupler was the only single-mode fiber coupler available. However its use was limited only to the laboratory environment, because it was very difficult to ruggedize the coupler either by an epoxy potting or thermal fusing of etched fibers. The outputs kept oscillating between fibers during the ruggedization process, and the final split ratio was rather unpredictable.
In the early 1980's, the single-mode fiber displaced the multimode fibers almost completely in the fiber optic market. The industry kept improving the biconical fused coupler technique until it became good enough even for the single-mode fibers. A series of invention disclosures have been made along the way, for example in U.S. Pat. Nos. 4,798,438, 4,842,359 of Imoto, et. al., and 4,961,617 of Shahidi, which use the fused-tapered technique in modified forms. However it has remained as a very delicate process, especially when the number of the input or output fibers exceeds two. The largest number of ports for single-mode fiber couplers available today have four inputs and four outputs, or so-called (4.times.4), and one input and seven outputs (1.times.7). To get a larger channel numbers, a number of (2.times.2) couplers are cascaded. This results in a extensive labor and high price.
The two kinds of single-mode fiber coupler embodiments mentioned above, the twist-and-etch coupler and the biconical fused coupler, which are sometimes called all-fiber couplers, inherently suffer from wavelength and polarization dependence of the coupling ratio due to the interferometric nature of the coupling mechanism. The dependence may be reduced by making the path lengths of the member fibers as equal as possible, and minimizing any factor that destroys the circular symmetry of the sectional shape or internal strain in the coupling region.
As an alternative approach, light coupling may be performed by optical channel waveguides fabricated on a bulk optical substrate. Examples of such a coupler design and its variations include U.S. Pat. Nos. 4,566,753 of L. Manscheke, 4,653,845 of Y. Tremblay and et. al., 4,904,042 of C. Dragone, 4,950,045 of T. Brichenno. This method of using planarized waveguides, being the most expensive way, is employed almost exclusively for fabricating single-mode fiber couplers with the number of input/output ports larger than (4.times.4) or (1.times.7). The production process involves the fabrication of the planarized channel waveguides, then cutting and polishing the end facets of the substrate to make them flat, smooth, and sharply cornered at the connecting interface, then aligning ten-micron fiber cores to the several-micron waveguides in an end-butt fashion with better than one or two micron accuracy, and then gluing down the fibers in the aligned positions, and making sure that the fibers do not move more than one or two microns while the glue is being cured. The difficulty of the tedious process steps is reflected in the high price of such a single-mode fiber star coupler.
Thus, there is a keen need to devise an embodiment for manufacturing multi-port fiber couplers, especially for the single-mode, that does not require these tedious fabrication and piecewise assembly steps.